The production of hydrogen is an increasingly common and important procedure in the world today. Production of hydrogen in the U.S. alone currently amounts to about 3 billion cubic feet per year, with output likely to increase. Uses for the produced hydrogen are varied, ranging from uses in welding, in production of hydrochloric acid, and for reduction of metallic ores. An increasingly important use of hydrogen, however, is the use of hydrogen in fuel cells or for combustion. This is directly related to the production of alternative fuels for machinery, such as motor vehicles. Successful use of hydrogen as an alternative fuel can provide substantial benefits to the world at large. This is possible not only because hydrogen is produced without dependence on the location of specific oils or other ground resources, but because burning hydrogen is atmospherically clean. Essentially, no carbon dioxide or greenhouse gasses are produced when burning hydrogen. Thus, production of hydrogen as a fuel source can have great impact on the world at large.
For instance, electrolysis, which generally involves the use of electricity to decompose water into hydrogen and oxygen, is a commonly used process. Significant energy, however, is required to produce the needed electricity to perform the process. Similarly, steam reforming is another expensive method requiring fossil fuels as an energy source. As could be readily understood, the environmental benefits of producing hydrogen are at least partially offset when using a process that uses pollution-causing fuels as an energy source for the production of hydrogen.
There is further need in environmental interests for new developments of biodegredation. Biodegredation refers to the degredation of sewages, effluents, toxic substances or other material organic material by microorganisms. The breakdown of toxic substances is also known as bioremeduiation. Biodegredation typically occurs in anarobic environments, and is generallys the process of converting organic materials back into CO2 and/or H2O through microbial action. Biodegredation is useful in that it breaks down unwanted or uneeded organic substances into natureal substances. However, a typical biodegradation product results in the formation of methane. Methane has a hgreenhouse gas having a high level of global warming potential. Excessive release of methane into the atmosphere is highly undesireable.
Thus, producing hydrogen from biological systems, through biodegradation or bioremediation, wherein the energy for the process is substantially provided by naturally occurring bacteria, is an optimal solution. Fermentation of organic matter by hydrogen producing microorganisms, such as Bacillus or Clostridium, is one such method. Nonetheless, hydrogen production relating to the above methods has remained problematic, and the need remains for the ability to optimize yields of hydrogen while minimizing expenditures.
New methods of hydrogen generation are needed. One possible method is to convert waste organic matter into hydrogen gas. Microbiologists have for many years known of organisms which generate hydrogen as a metabolic by-product. Two reviews of this body of knowledge are Kosaric and Lyng (1988) and Nandi and Sengupta (1998). Among the various organisms mentioned, the heterotrophic facultative anaerobes are of interest in this study, particularly those in the group known as the enteric bacteria. Within this group are the mixed-acid fermenters, whose most well known member is Escherichia coli. While fermenting glucose, these bacteria split the glucose molecule forming two moles of pyruvate (Equation 1); an acetyl group is stripped from each pyruvate fragment leaving formic acid (Equation 2), which is then cleaved into equal amounts of carbon dioxide and hydrogen as shown in simplified form below (Equation 3).Glucose→2Pyruvate  (1)2Pyruvate+2Coenzyme A→2Acetyl-CoA+2HCOOH  (2)2HCOOH→2H2+2CO2  (3)
Thus, during this process, one mole of glucose produces two moles of hydrogen gas. Also produced during the process are acetic and lactic acids, and minor amounts of succinic acid and ethanol. Other enteric bacteria (the 2, 3 butanediol fermenters) use a different enzyme pathway which causes additional CO2 generation resulting in a 6:1 ratio of carbon dioxide to hydrogen production (Madigan et al., 1997).
There are many sources of waste organic matter which could serve as a substrate for this microbial process, namely as a provider of pyruvate. One such attractive material would be organic-rich industrial wastewaters, particularly sugar-rich waters, such as fruit and vegetable processing wastes. In additional embodiments, wastewaters rich not only in sugars but also in protein and fats could be used, such as milk product wastes. The most complex potential source of energy for this process would be sewage-related wastes, such as municipal sewage sludge and animal manures.
The creation of a gas product that includes hydrogen can be achieved in a bioreactor, wherein hydrogen producing microorganisms and a food source are held in a reactor environment favorable to hydrogen production. Substantial, systematic and useful creation of hydrogen gas from microorganisms, however, is problematic. The primary obstacle to sustained production of useful quantities of hydrogen by microorganisms has been the eventual stoppage of hydrogen production, generally coinciding with the appearance of methane. This occurs when methanogenic bacteria invade the reactor environment converting hydrogen to methane, typically under the reaction CO2+4H2→CH4+2H2O. This process occurs naturally in anaerobic environments such as marshes, swamps, pond sediments, and human intestines.
It is of further importance to increase the number of hydrogen producing microorganisms in a system to the point that fixed colonies of biofilm are existent in the bioreactor. Increasing the number of hydrogen producing microorganisms and biofilm and thereby increasing the overall percentage of hydrogen producing microorganisms is beneficial, particularly in large scale reactors. Therefore, it is important to create a bioreactor environment that is conducive to hydrogen producing microorganism growth and maintenance in addition to hydrogen production.
Thus, there continually remains a need to produce substantial and useful levels of hydrogen in an a system that provides an environment conducive to metabolism of organic feed material by hydrogen producing microorganisms.